VAEIABLE EESEMBLANCE IN VERTEBKATA, ETC. 107 



Variable Resemblance of northern animals most nearly 

 related to that of certain insects 



The acquisition of a special winter covering as a 

 response to the stimulus of cold is most nearly related 

 to the Variable Eesemblance exhibited by many cater- 

 pillars and chrysalides. It will be shown in the next 

 chapter that these latter changes are similar to the 

 above in that the stimulus (of reflected light) acts 

 upon the skin ; that the results are in all probability 

 indirect, and take place through the part of the 

 nervous system which regulates the production of 

 colour ; finally, that far greater time is required for 

 the accomplishment of the change than in those 

 animals in which the stimulus acts upon the eye, and 

 in which existing pigments are arranged instead of new 

 substances elaborated. 



Rapid adjustment of Colour in certain invertebrate 

 animals 



Certain invertebrate animals, however, possess the 

 power of rapidly adjusting their colour to that of their 

 surroundings. It is well-known in Crustacea, and is 

 probably very common among them. The power has 

 been proved to depend upon the eye as among the 

 vertebrates. Some cuttle-fish also can modify their 

 colours in the same manner, with remarkable pre- 



